OKUMA, Japan, March 8 (Reuters) - A costly "ice wall" is
failing to keep groundwater from seeping into the stricken
Fukushima Dai-ichi nuclear plant, data from operator Tokyo
Electric Power Co shows, preventing it from removing radioactive
melted fuel at the site seven years after the disaster.

When the ice wall was announced in 2013, Tepco
assured skeptics that it would limit the flow of groundwater
into the plant's basements, where it mixes with highly
radioactive debris from the site's reactors, to "nearly
nothing."

However, since the ice wall became fully operational at the
end of August, an average of 141 metric tonnes a day of water
has seeped into the reactor and turbine areas, more than the
average of 132 metric tonnes a day during the prior nine months,
a Reuters analysis of the Tepco data showed.

The groundwater seepage has delayed Tepco's clean-up at the
site and may undermine the entire decommissioning process for
the plant, which was battered by a tsunami seven years ago this
Sunday. Waves knocked out power and triggered meltdowns at three
of the site's six reactors that spewed radiation, forcing
160,000 residents to flee, many of whom have not returned to
this once-fertile coast.

Though called an ice wall, Tepco has attempted to create
something more like a frozen soil barrier.

Using 34.5 billion yen ($324 million) in public funds, Tepco
sunk about 1,500 tubes filled with brine to a depth of 30 meters
(100 feet) in a 1.5-kilometer (1-mile) perimeter around four of
the plant's reactors. It then cools the brine to minus 30
degrees Celsius (minus 22 Fahrenheit).

The aim is to freeze the soil into a solid mass that blocks
groundwater flowing from the hills west of the plant to the
coast.

However, the continuing seepage has created vast amounts of
toxic water that Tepco must pump out, decontaminate and store in
tanks at Fukushima that now number 1,000, holding 1 million
tonnes. It says it will run out of space by early 2021.

"I believe the ice wall was 'oversold' in that it would
solve all the release and storage concerns," said Dale Klein,
the former chairman of the U.S. Nuclear Regulatory Commission
and the head of an external committee advising Tepco on safety
issues.

"The hydrology of the Fukushima site is very complicated and
thus the exact water flow is hard to predict," he said,
"especially during heavy rains."

TYPHOON

The water inflows often fluctuate with rainfall. The dry
month of January averaged 83 tons a day, Tepco data showed.

But when a typhoon struck during the last week of October,
866 tons a day poured into the reactors.

Overall, Tepco says a combination of drains, pumps and the
ice wall has cut water flows by three-quarters, from 490 tons a
day during the December 2015 to February 2016 period to an
average of 110 tons a day for December 2017 to February 2018.

It is hard to measure exactly how much the ice wall is
contributing, Tepco officials say, but based on computer
analysis the utility estimates the barrier is reducing water
flows by about 95 tonnes a day compared to two years ago, before
the barrier was operating.

"Our assessment is that the ice wall has been effective,"
said Naohiro Masuda, Tepco's chief decommissioning officer,
adding that rain falling within the ice wall perimeter
contributed to surging volumes. “We now believe we have a system
in place to manage the water level."

However, a government-commissioned panel on Wednesday
offered a mixed assessment of the ice wall, saying it was
partially effective but more steps were needed.

Controlling the groundwater seepage using the ice wall has
been central to Japan's program to show it had the Fukushima
decommissioning in hand.

The barrier was announced just days before Tokyo won the bid
to host the 2020 Summer Olympics and Prime Minister Shinzo Abe
declared that Fukushima was "under control" in his final pitch
to the International Olympic Committee.

In addition to the building costs, the ice wall needs an
estimated 44 million kilowatt hours of electricity a year to
run, enough to power about 15,000 typical Japanese homes.

NO MORE SPACE

Meanwhile, Tepco must decide how to cope with the growing
volume of water stored on site.

The purification process removes 62 radioactive elements
from the contaminated water but it leaves tritium, a mildly
radioactive element that is difficult to separate from water.
Not considered harmful in low doses, tritium is released into
oceans and rivers by nuclear plants around the world at various
national standard levels.

But local residents, particularly fishermen, oppose ocean
releases because they fear it will keep consumers from buying
Fukushima products. Many countries, including South Korea and
China, still have restrictions on produce from Fukushima and
neighboring areas.

A government-commissioned task force is examining five
options for disposing of the tritium-laced water, including
ocean releases, though no decision has been made.

Ken Buesseler, a radiochemist at the Woods Hole
Oceanographic Institution in the United States, suggests that
Tepco should open the tanks to external inspections to see if
the water is safe.

"From the public's viewpoint, I think they'd want a bit of
independent confirmation," Buesseler said. "It's no harder and a
lot cheaper than building an ice wall."
($1 = 106.0900 yen)